A comprehensive description for the constitutive behavior of polycrystalline shape memory alloys (SMAs) is proposed based on the concept that a SMA is dynamically composed of austenite and martensite, the constitutive behavior of which is a dynamic combination of the individual behavior of each of the two phases. In the interested ranges of stress and temperature the behavior of austenite is assumed to be linearly elastic while that of martensite elastoplastic. The main features of SMAs such as ferroelasticity at lower temperature, shape memory effect (SME), pseudoelasticity at higher temperature and other typical features can be successfully replicated with the proposed constitutive model. The ferroelasticity, pseudoelasticity and SME of SMA Au-7.5 at.% Cd subjected to uniaxial forward and reverse loading and the pseudoelasticity of Cu Alan Mn SMA polycrystal subjected to proportional and nonproportional complex stress or strain histories are analyzed and compared with the experimental results.